My understanding of this - and perhaps this is what Andrew is getting at - the SHAPE of the colour space is as important as its size. The shape of the gamut the printer can reproduce is different from the shape of ARGB98. Hence if you confine yourself to ARGB98 you will miss capturing some colours that the printer could otherwise reproduce. Therefore using a colour space for the file that is for all colours larger than the printer colour space ensures that you will serve-up to the printer every colour it can reproduce.
There is, however, a large literature on the web, particularly in Dan Margulis' Color Theory List, debating the merits of using these wide colour spaces. From my reading of this extensive and arcane debate, I believe the centerweight of these arguments seem to still favour using the larger colour spaces, but being aware that under some conditions it may be necessary to "tame" certain hues that go way out of gamut. Andrew please chime in if I got this right (or wrong)..........................